In [1] he can not detect the Varroa within the hive, nevertheless he notice the behavior of the hive is as if it had it. In [2] the hive is already dead, then is when he find the Varroa. In the comments on [2], one beekeeper explains that when the combs are twisted the mites fall into the combs rather than onto the floor which is traditionally used to detect them ( The sugar [3] or CO2 technique to detect Varroa in any type of hive is recommended by other beekeepers in the comments).
[1] https://www.youtube.com/watch?v=iYKL7hrp23k HIIVE Confusion
[2] https://www.youtube.com/watch?v=YsdHyRdpfB0 All the bees dead - why Varroa was so treacherous here
[3] By comments on other videos about the topic, this needs around 200 bees which are placed in a container with grids to which sugar is added. When shaken, the Verroa falls and a count can be made. The topology of this hive makes it difficult to gather this amount of bees (in the video [2] one can see that the hive would have to be dismantled).